This invention relates to an electric circuit comprising an oscillator circuit which comprises a capacitive element, a first and a second electrode of which
are coupled to a reference node each via a respective load circuit, and are also PA1 switchably coupled to the reference node via a first and a second voltage source, respectively, PA1 and are also coupled to respective inputs of a switching control circuit which is coupled to the first and the second voltage source, PA1 for controlling a second switching operation where the second electrode is uncoupled from the second voltage source and the first electrode is coupled to the first voltage source, PA1 a first voltage difference is imposed between voltages supplied by the first voltage source directly before the first switching operation and by the second voltage source directly after the first switching operation, respectively, and PA1 a second voltage difference is imposed between voltages supplied by the first voltage source directly after the second switching operation and by the second voltage source directly before the second switching operation, respectively,
for controlling a first switching operation where the first electrode is uncoupled from the first voltage source and the second electrode is coupled to the second voltage source, and PA2 the first and the second switching operation being executed when a capacitance voltage between the first and the second electrode exceeds an upper threshold and drops below a lower threshold, respectively.
An electric circuit of this kind is known from EP-A 0296668, which corresponds to U.S. Pat. No. 4,871,985 (Oct. 3, 1989) and is often used as a local oscillator. In the context of the present Application, a local oscillator is to be understood to mean an oscillator whose oscillation signal, for example as available on the first electrode, is utilized in only a pan of the electric circuit, for example, in a receiver in which it is to be confined to the mixing stage.
The described oscillator is based on the periodic charging and discharging of the capacitive element. This is realised as follows. After the second switching operation, the capacitive element and the load circuit coupled to the second electrode are connected in a first series connection across the first voltage source. A current generated through this first series connection by the first voltage source will charge the capacitive element, giving rise to an increasing voltage between the electrodes of the capacitive element (this voltage will be referred to hereinafter as the capacitance voltage).
When this capacitance voltage exceeds the upper threshold, the first switching operation takes place. The first voltage source is then uncoupled. At the same time, or at least at a later instant, the second voltage source is coupled on and a second series connection, consisting of the capacitive element and the load circuit coupled to the first electrode, is thus connected across the second voltage source. Each of the first and second series connections thus comprises the capacitive element and a load circuit. The difference, however, consists in that the capacitive element in the first and the second series connection is connected to the first and the second voltage source, respectively, via opposite electrodes. Therefore, the second voltage source generates a current which traverses the capacitive element in the opposite direction relative to the current generated by the first voltage source. The current through the second series connection, therefore, will discharge the capacitive element again, resulting in a decreasing capacitance voltage. When this voltage drops below the lower threshold, the second operation takes place and the charging process will be repeated. Periodic charging and discharging are thus achieved.
The voltage at the first electrode is determined alternately by the first voltage source and, via the capacitive element, by the second voltage source. In the circuit disclosed in EP-A 0296668, the voltages delivered by the first and the second voltage source are equal. Therefore, the switching operation will give rise to a voltage transient at the first electrode whose magnitude corresponds to the voltage present across the capacitive element due to the charge built up.
This is an adverse effect in view of parasitic couplings in the circuit, causing the high-frequency components of the oscillator signal to be radiated beyond the part of the circuit served by the oscillator. Because of their high frequency content, the voltage transients will contribute much more to such radiation than the normal increasing and decreasing oscillator voltages during charging and discharging.